Saving as .puffin _sometimes_ breaks

My repro: connect `puffin_viewer` to a server with `--url 127.0.0.1:8585`, hit save, then try to load the saved `.puffin` in a new `puffin_viewer` process.

This _sometimes_ works, but sometimes I end up with a broken `.puffin` file, like this:

[broken.puffin.zip](https://github.com/user-attachments/files/21604643/broken.puffin.zip)

* Probably broke in https://github.com/EmbarkStudios/puffin/pull/169
* Maybe related to https://github.com/EmbarkStudios/puffin/issues/200 ?